Carcinoembryonic antigen (CEA) is a member of the immunoglobulin superfamily and is composed of seven domains linked to the cell membrane through a glycosylphosphatidylinositol anchor.
Anti-tumor monoclonal antibodies have much clinical potential as both therapeutic and diagnostic agents. For this reason, monoclonal antibodies raised against carcinoembryonic antigen (CEA) have been generated to detect various epitopes on CEA (Muraro et al., Cancer Res., 45:5769-5780, 1985, herein incorporated by reference). These antibodies have been designated COL-1 through COL-15 (Muraro et al., Cancer Res., 45:5769-5780, 1985; Ohuchi et al., Cancer Res. 47:3565-3571, 1987; Wilkinson et al., Proc. Natl. Acad. Sci. 98:10256, 2001) and have been instrumental in identifying the differential expression pattern of CEA in different tissues and in numerous carcinomas, such as gastrointestinal, colorectal, breast, ovarian, and lung carcinomas. Of these monoclonal antibodies, COL-1 is of clinical importance because it has both a high affinity for CEA and does not cross-react with other members of the immunoglobulin superfamily.
Despite the high binding affinity demonstrated by murine monoclonal antibodies, the administration of these antibodies to humans can be limited by their inherent immunogenicity and the development of a human anti-murine antibody (HAMA) response in these patients. The HAMA response can involve allergic reactions and an increased rate of clearance of the administered antibody from the serum. Many attempts have been made to design murine monoclonal antibodies that are less immunogenic to humans by “humanizing” the antibodies. Such attempts first involved the design of human-mouse chimeras, in which a murine antigen-binding variable region is coupled to a human constant domain. Other, more recent, strategies have resulted in the design of humanized antibodies with the retention of fewer murine residues. These strategies include the development of complementarity determining region (CDR)-grafted monoclonal antibodies and specificity-determining residue (SDR)-grafted monoclonal antibodies, where CDRs and/or SDRs are grafted onto the variable light (VL) and/or variable heavy (VH) framework (FR) of human monoclonal antibodies. Using these techniques the number of immunogenic murine residues retained in these antibodies is reduced. A few framework residues considered crucial for the maintenance of antigen binding are also grafted onto the human frameworks. These antibodies can still evoke an undesired anti-variable region response against the potentially immunogenic framework residues. Thus, there exists a need to develop a humanized COL-1 antibody with both high affinity and reduced immunogenicity for use in human subjects.